The invention relates to level measuring devices operating with microwaves. Such measuring devices that operate without physical contact are used in numerous branches of industry. In many industrial applications, for example in the oil industry, highly accurate measurement, for example 1 mm, is required.
For measuring the level microwaves are transmitted by means of an antenna to a surface of a filling material substance and an echo signal reflected at the surface is received. An echo function representing the echo amplitudes as a function of the distance is formed and is used to determine the probably useful echo and its transit time. The transit time is used to determine the distance between the surface of the filling material and the antenna.
One method of measuring distances by means of reflected microwaves is pulsed radar. In the case of pulsed radar, short microwave pulses are transmitted periodically, reflected by the surface of the filling material and received again after a distance-dependent transit time. The received signal amplitude as a function of the time represents the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a particular distance from the antenna.
A level measuring device is described in U.S. Pat. No. 4,521,778 (counterpart to DE-A 31 07 444) that operates with microwaves and which comprises:
a transmission pulse generator for generating microwave pulses at a transmission repetition rate f.sub.s which are transmitted during operation in the direction of a filling level to be measured by means of an antenna are reflected at a surface of the filling material and the echo signal of which is received, PA1 a reference pulse generator for generating microwave pulses which are identical to the microwave pulses generated by the transmission pulse generator but have a reference repetition rate f.sub.r differing from the transmission repetition rate f.sub.s by a small frequency difference, PA1 a first mixer, PA1 to the first input of which is applied an output signal of the reference pulse generator, PA1 to the second input of which is applied the echo signal and PA1 at the output of which an intermediate frequency signal appears that, with respect to the echo signal, is slowed down by a time dilation factor equal to a quotient of the transmission repetition rate and the frequency difference. PA1 .DELTA.T is the time difference between two points of the intermediate frequency signal, PA1 .DELTA.t is the actual time difference between the associated points of the echo signal, PA1 f.sub.s is the transmission repetition rate and PA1 .DELTA.f is the differential frequency (f.sub.s -f.sub.r) of transmission repetition rate f.sub.s and reference repetition rate f.sub.r PA1 a transmission pulse generator for generating microwave pulses at a transmission repetition rate which are transmitted during operation by means of an antenna in the direction of a filling level to be measured, are reflected at a surface of the filling material and the echo signal of which is received, PA1 a reference pulse generator for generating microwave pulses which are identical to the microwave pulses generated by the transmission pulse generator but have a reference repetition rate different from the transmission repetition rate by a small frequency difference, PA1 a first mixer, PA1 to the first input of which is applied the echo signal, PA1 to the second input of which is applied an output signal of the reference pulse generator, and PA1 at the output of which appears an intermediate frequency signal that, with respect to the echo signal, is slowed down by a time dilation factor which is equal to a quotient of the transmission repetition rate and the frequency difference, PA1 a first subcircuit for sampling the intermediate frequency signal, PA1 in which the time interval between two successive samplings is equal to a time unit, and PA1 means for generating a differential signal which changes with the differential frequency, PA1 a second subcircuit for determining its period duration on the basis of the time unit, and PA1 an evaluation unit, PA1 which determines the time dilation factor in the time unit from the period duration, and PA1 which by means of this time dilation factor in the time unit determines from a time interval present in the time unit between two sampling points of the intermediate frequency signal an actual time interval between two points of the echo signal that corresponds to the two sampling points. PA1 to the first input of which is applied a signal PA1 which changes at the transmission repetition rate and PA1 to the second input of which is applied a signal PA1 which changes at the reference repetition rate and PA1 at the output of which appears the differential signal. PA1 an oscillator, the frequency of which is equal to the transmission repetition rate, PA1 a pulse generator, which is connected downstream of the oscillator, PA1 and which converts an output signal of the oscillator into a control signal, and PA1 a microwave source, which is controlled by the control signals, and which transmits the microwave pulses. PA1 an oscillator, the frequency of which is equal to the reference repetition rate, PA1 a pulse generator, which is connected downstream of the oscillator, PA1 and which converts an output signal of the oscillator into a control signal, and PA1 a microwave source, which is controlled by the control signals, and which transmits the microwave pulses. PA1 a comparator, PA1 at the output of which appears a signal at a first signal level if the differential signal at the sampling instant exceeds a prescribed threshold value and PA1 at the output of which appears a signal at a second signal level if the differential signal at the sampling instant is less than a prescribed threshold value, PA1 an edge detector, which is connected to a comparator PA1 and detects the beginning of a period of the differential signal, and PA1 a counter, PA1 which counts the number of elapsing time units during a predefined number of periods of the differential signal.
The intermediate frequency signal has the same characteristics as the echo signal, but is extended with respect to the echo signal by the time dilation factor. At a transmission repetition rate of several megahertz, a frequency difference of, for example, 10 Hz to 100 Hz and a microwave frequency of several gigahertz, the frequency of the intermediate frequency signal is below 100 kHz.
This allows the use of very much slower and cost-saving components for signal pickup and/or signal evaluation.
The intermediate frequency signal is usually demodulated and sampled and an envelope curve is created from the sampled values and stored in a memory, where it is available for further processing. The envelope curve typically has a maximum that depends of the reflection of the microwave pulse at the surface of the filling material. The time interval between a reference instant and the maximum of the intermediate frequency signal and the time dilation factor can be used to determine the actual transit time of the microwave pulse to the surface of the filling material and back. This actual transit time is a measure of the distance between the surface of the filling material and the filling level measuring device.
The following relationship exists for the time difference .DELTA.T between two points of the intermediate frequency signal and the actual time difference .DELTA.t between the associated points of the echo signal: EQU .DELTA.T=(f.sub.s /.DELTA.f).DELTA.t. (1)
where:
and f.sub.s /.DELTA.f is equal to the time dilation factor.
The accuracy of the filling level measurement consequently depends on how exactly the variables .DELTA.T, f.sub.s and .DELTA.f are known.